1. Field of the Invention
The present invention relates to a laser microscope apparatus.
This application is based on Japanese Patent Application No. 2008-256253, the content of which is incorporated herein by reference.
2. Description of Related Art
There is a known laser microscope apparatus in the related art which is provided with two light paths via which laser light in different frequency bands is guided and a dichroic mirror that reflects the laser light in one light path and transmits the laser light in the other light path to combine the two light paths (for example, see Japanese Unexamined Patent Application, Publication No. 2004-86009). In this laser microscope apparatus, a specimen is irradiated with infrared pulsed laser light being used as the laser light guided via the first light path, and fluorescence produced by a multiphoton excitation effect is observed. An excitation dichroic mirror that reflects laser wavelengths and transmits the fluorescence coming from the specimen is provided in the combined laser light path.
In the laser microscope apparatus described above, normally the excitation dichroic mirror is designed to reflect the laser light and transmit the fluorescence from the specimen. By doing so, because the reflectance characteristic of the interference film used in the reflecting surface of the dichroic mirror allows reflection of only the narrow-wavelength-band laser light and allows transmission of the rest of the band containing the fluorescence, the design of the interference film can be simplified.
However, because the interference film of the dichroic mirror is constructed of a multilayer film, in the reflection light path, the group velocity dispersion is large only in a particular wavelength region. This is influenced by the fact that the optical path length for each wavelength varies greatly due to the reflection at each layer in the multilayer film. Ultrashort pulsed laser light with a pulse width of approximately 100 fs used in multiphoton-fluorescence observation normally has a wavelength band of about 10 nm; therefore, if the wavelength region where the group velocity dispersion is large is contained within this wavelength range, the ultrashort pulsed laser light reflected at this multilayer film experiences a large chirp, increasing the pulse width. Moreover, because this chirp is a nonlinear chirp, it cannot be compensated for with a linear chirp correction method using, for example, a prism pair. Therefore, because the increase in pulse width cannot be compensated for, there is a drawback in that it is not possible to efficiently generate the multiphoton excitation effect.